This invention relates to new and improved voltage controlled oscillator circuits and more particularly to means for supplying the oscillation output signal to a phase comparator in a phase locked loop, especially-although not exclusively for demodulation of angle-modulated waves.
In general, a voltage controlled oscillator (hereinafter referred to as a VCO) for use in a phase locked loop (hereinafter referred to as a PLL) is required to possess desirable properties, such as a linear relationship between an outside control voltage and the output frequency. The VCO should also have equal duty cycles in its output oscillation waveform.
Furthermore, if a VCO is to be incorporated in a monolithic integrated circuit (IC), the VCO is required to have characteristics such as an output voltage which is not extremely high and an output waveform which does not contain a large quantity of harmonic components. That is, not the waveform should have a sharp-cornered rectangular form, but should have rounded-off corners. These rounded corners prevent the VCO output from imposing interference on other circuits within the same IC chip.
A further requirement, if the VCO is to be incorporated in a monolithic IC, the the current consumed from the +B power source should be unvarying, with respect to the oscillation output, in order to prevent the oscillation current of the VCO from affecting other circuits via the power supply circuit.
A novel voltage controlled oscillator, which satisfies the above stated requirements, is described in U.S. patent application Ser. No. 417,475, filed on Nov. 19, 1973, and entitled "Circuit arrangement of voltage controlled oscillator", now U.S. Pat. No. 3,886,485.
A so-called discrete four-channel record disc, which has been described in U.S. Pat. No. 3,686,471, is at present being widely used. In the process of recording on this disc, a direct wave of a sum signal of two of the channels and an angle modulated wave obtained by angle modulating a carrier wave of 30 KHz by means of a difference signal of two of the channels are multiplexed and recorded on the left and right walls of the disc sound groove. For the demodulation of the angle modulated wave reproduced from this four-channel record disc, a PLL circuit is generally used.
Next to be considered is the effect which occurs when a reproducing (playing) pickup stylus is placed on a stopped discrete four-channel record disc, which is then starting to rotate. In this case, the record disc starts rotating from its stopped position and gradually increases its rotational speed until, after a specific time, it reaches a constant rotational speed. Since the pickup stylus is riding on the record disc during this period of less than normal speed, a previously recorded signal is being reproduced during this period.
This reproduced signal is at a lower frequency than the normal frequency. Consequently, erroneous locking with respect also to a signal of a frequency of the order a fraction of the carrier wave frequency, for example, tends to occur in the PLL circuit. This mislocking causes noise.
Accordingly, the generation of this noise can be prevented by previously preventing the oscillation frequency of the voltage controlled oscillator from decreasing to a value below a certain critical frequency. After the rotational speed becomes close to the normal, the phase of the oscillation signal of the voltage controlled oscillator is locked to the phase of the input angle modulated signal.
This voltage controlled oscillator of U.S. Pat. No. 3,686,471 is so adapted that the oscillation frequency will not become less than a certain critical frequency. However, it is difficult to hold this critical oscillation frequency at a specifically constant value--; for reasons such as deviations in the current amplification of transistors and in the resistance values of the resistors used.
Another voltage controlled oscillator, which adds circuits that solve these problems, is found in the U.S. Pat. No. 3,870,971, entitled "Circuit Arrangement of Voltage Controlled Oscillator".
According to this patent the voltage controlled oscillator responds to a control current having a linearly proportional relationship with the oscillation frequency. Moreover, the oscillation frequency is controlled by the current, which varies within a predetermined range with respect to a specific current as a center. Thus, a critical value of the oscillation frequency is accurately held at a predetermined frequency.
However, in this circuit of U.S. Pat. No. 3,870,971, if the power supply voltage fluctuates, the bias current varies, since it is dependent on the power supply voltage. For example, if the power supply voltage rises, the free-running frequency of the voltage controlled oscillator also increases unavoidably, which has been a problem.